This invention relates to the measurement of density, and more particularly to a test instrument and method for measuring the density of a sample using gamma radiation. The invention is especially suited for measuring the density in a relatively thin zone below the surface of a sample.
In the asphalt pavement construction industry, portable nuclear gauges are frequently used for measuring the density of the asphalt pavement. Often, the asphalt paving material is applied in relatively thin layers, e.g. on the order of about one to two inches in thickness, over a prepared roadbed foundation or an existing paved roadway. Consequently, there is a need to measure density of the pavement sample in a relatively thin zone, e.g., one to three inches in depth, below the pavement surface. To this end, nuclear density gauges have been developed for directly measuring the density of a thin layer of paving material. For example, nuclear xe2x80x9cthin layerxe2x80x9d gauges of this type are described in commonly owned U.S. Pat. Nos. 4,641,030; 4,701,868 and 6,310,936. The gauges described in these patents use a Cesium-137 (137Cs) source of gamma radiation containing approximately eight millicuries of Cesium-137. Gamma radiation that is Compton scattered from the underlying sample is detected by Geiger-Mueller tubes positioned to form two geometrically differing source-to-detector relationships, and the density of the material is calculated based upon the gamma radiation counts detected by the respective detectors.
Although the activity of the gamma radiation source in these gauges is quite small, in the millicurie range, and can be safely used by an operator with ordinary precautions and care, regulatory agencies impose restrictions on the handling, transport, storage and use of such gauges, and on persons qualified to operate such gauges. Consequently, there exists a need for a gauge which uses a radiation source of a much lower activity level which is not subject to the regulatory requirements of existing gauges.
It is therefore an object of the present invention to provide a nuclear gauge suited for measuring the density in a relatively thin zone below the surface of a sample, and which uses a low activity radiation source.
It is a more specific object of the present invention to provide a gauge which can operate using a gamma radiation source having an activity in the microcurie range, and more specifically with an activity of no more than 100 microcurie, and more desirably an activity of no more than 50 microcurie. Gauges employing these low activity nuclear sources are subject to fewer and less stringent restrictions and regulations, if any.
Prior attempts to produce nuclear gauges using low activity (microcurie) radiation sources have had limited success, primarily because of their limited levels of accuracy. By way of example, one prior nuclear gauge using a low activity nuclear source is described in commonly owned U.S. Pat. No. 4,766,319. The main difficulty in developing a gauge based on a low activity gamma radiation source is that the signal to noise ratio of the gamma radiation detection is low because of the relatively low gamma radiation flux from a low activity source. Background radiation from certain naturally occurring radioactive elements (e.g. K-40, U and Th) present in the material to be tested generate noise which cannot be ignored without sacrificing the accuracy of measurement. With conventional gauges using higher activity gamma radiation sources (e.g. a 8000 microcurie Cs-137 source), the signal to noise ratio is high and the background radiation does not contribute significant error.
The present invention provides a nuclear density gauge and method which is suited for measuring the density in a relatively thin zone beneath the surface of a sample of paving material. The gauge may be designed to measure the density in a zone up to a specific depth of, for example, up to 1 or as much as 3 inches beneath the surface of the material sample. The gauge uses one or more gamma radiation sources having a total activity of no more than 100 microcurie. The gauge includes a gauge housing having a surface adapted to be positioned on a surface of the material sample. The microcurie gamma radiation source is mounted within the housing for emitting gamma radiation through the base and into an underlying material sample. At least one energy selective gamma radiation detector is mounted within the gauge housing in spaced apart relation with respect to the gamma radiation source, with the detector being operable for producing signals representing the energy level of the detected gamma radiation. Density calculating means is connected to the detector and is operable for calculating a value for the density of the material based upon detected signals having an energy level within a predetermined portion of the energy spectrum of the gamma radiation detected by the detector. In one embodiment, the density calculating means includes an analyzer which is connected to the detector and is operable for classifying and accumulating signals from the detector into one or more channels corresponding to said predetermined portion of the energy spectrum. The analyzer may, for example, comprise a multichannel analyzer which classifies and accumulates signals in a plurality of discrete channels over the energy spectrum of the gamma radiation detected by the detector, and wherein at least one of these discrete channels defines said predetermined portion of the energy spectrum.
In one specific embodiment, the predetermined portion of the energy spectrum which is used for density calculation has a lower limit of 0.1 MeV or greater and an upper limit which is less than the characteristic primary energy of the source. The gamma radiation source may comprise at least one Cesium-137 gamma radiation source with a 0.662 MeV primary energy. Preferably, the detector is a scintillation detector, and the system may include an analyzer connected to the scintillation detector which is capable of identifying the counts which have an energy within the specified energy spectrum.